1. Field of the Invention
The invention is in the field of light sources providing visible light for display and/or illumination purposes and more particularly relates to a semiconductor light source.
2. Description of Related Art
Semiconductor light sources offer a much longer lifetime compared to conventional lamps, smaller form factor and better energy conversion efficiency, suggesting lower power bills. Among semiconductor sources of visible light, Light Emitting Diodes (LEDs) and Laser Diodes (LDs) are well-known and popular. All state of the art semiconductor-based light sources on the market can be classified in these two main categories:
light emitting diodes (LEDs), produce an incoherent light with a relatively broad spectrum.
light amplifiers by means of stimulated emission of radiation (lasers), produce a narrow spectrum and highly coherent light beam; a narrow waveguide confines the electromagnetic radiation resulting in good in-fiber coupling of the light, i.e. high spatial coherence.
LEDs are popular for illumination as well as for traffic lights, low power displays etc. LEDs however, offer a comparably small optical output power only. To overcome this problem, the emitting area is usually enlarged, increasing the surface of the device. However, for applications requiring a collimation of the light beam, this is disadvantageous. The large emitting area results in problems with collimating the emitted light beam, and the beam quality deteriorates with the die size. Therefore, light cannot be efficiently coupled to an optical element such as a waveguide or a light modulator etc. Optical output power is another concern. The current state-of-the-art of high brightness LED suggests that a device pushing 1,000 lumens out at the lens is not foreseeable for the next 4-5 years. This means that especially high output power projection devices will not be realizeable using LED light sources.
Laser diodes (LDs), however, offer high power, compact design, good energy conversion into light and good coupling efficiency to external optical systems, i.e. high spatial coherence. LDs have a well-defined emission wavelength and can be designed for different wavelengths of the optical spectrum. As every color (including white) the human can perceive may be represented by a superposition of light contributions in three main wavelengths (446 nm, 532 nm and 629 nm), LDs seem an ideal candidate to generate color displays by just combining three laser sources. With this technique, it is ideally possible to complete the whole color gamut.
However, in practice it has been found that the color impression from laser displays is not perceived to be ideal by humans. For this reason, energy consuming broadband light sources such as Xenon lamps still prevail on the market of display light sources. A further problem of laser light for displays is speckle formation. Speckles are due to an interference effect caused by the high coherence of light sources and cause distortions of the resulting displayed image. State of the art laser display systems for this reason include laborious and costly means for reducing light coherence such as light tunnels. Yet another disadvantage of laser displays is the polarization of the output light that for many display applications is undesired. Therefore, laser displays often include depolarizers.
As semiconductor materials for blue and green LEDs and LDs, GaN based compounds have been proposed. As an example, US patent application publication 2005/0127394 discloses nitride semiconductor devices with an active layer and super lattice cladding layers. In the publication, the structure is mentioned to be suitable for both, LEDs and LDs as well as superluminescent light emitting diodes, which as such are known for the generation of infrared radiation with a high linewidth or as (less ideal) replacement of lasers at wavelengths, where no suitable mirrors are available.